This is a Pearce Grip known as the Grip Ext, model PGG4PLUS.
This unit adds capacity to Generation 4 & 5 Sub-Compact" Mid" and Full Size Glock high capacity magazines. This magazine extension replaces the original floor plate and internal locking plate and adds two rounds to a 9mm; two rounds to a 40SW and 357SIG" and one round to a 45GAP magazine. This unit will not fit the 10mm or 45ACP models.
Features:
High Impact Polymer
Increased Comfort and Control
For High Capacity Metal Lined Glock Magazines